1. Field of the Invention
The present invention relates to a vibration plate, a speaker unit and a portable information terminal, and particularly to a vibration plate having an edge portion, a speaker unit provided with the vibration plate, and a portable information terminal.
2. Description of the Related Art
A speaker unit is mounted in a portable information terminal such as a mobile phone, a digital camera, a personal computer, a game machine, and a PDA (Personal Digital Assistant). The speaker unit includes a vibration plate. In addition, the speaker unit includes the so-called speaker and receiver.
It is required to decrease the lowest resonance frequency in order to improve the performance of the speaker unit. For example, there are the following three methods of decreasing the lowest resonance frequency. The first method is to reduce the thickness of the vibration plate. The second method is to increase the radius of curvature of the edge portion. The third method is to increase the mass of the vibration system which is the total mass of the vibration plate and the voice coil.
Furthermore, for example, Japanese Patent Laying-Open No. 2006-229657 discloses another method of decreasing the lowest resonance frequency by which the center portion and the edge portion of the vibration plate are formed from different members. This patent literature discloses that the center vibration portion and the annular vibration portion (edge portion) of a diaphragm (vibration plate) are formed from different members to enhance the rigidity of the center vibration portion while lowering the rigidity of the annular vibration portion, thereby preventing abnormal vibration from occurring in the center vibration portion, but allowing a margin of vibration limit to be provided in the annular vibration portion for suppressing an increase in the lowest resonance frequency.
In the first method as described above, the vibration plate having a reduced thickness causes a decrease in the strength of the vibration plate. Consequently, distortion may be increased to produce unusual noise. Therefore, there is a limit to reducing the thickness of the vibration plate for maintaining the basic sound quality of the speaker unit. Accordingly, the method of reducing the thickness of the vibration plate cannot allow the lowest resonance frequency to be decreased below a certain level.
The second method as described above requires the speaker unit to be reduced in size, which imposes a limit on the outer diameter of the vibration plate. When the radius of curvature of the vibration plate is increased within the size limitations, the shape of the vibration plate in the radial direction becomes nearly a straight line, which leads to a decrease in the line length of the vibration plate in the radial direction. In the state where the line length of the vibration plate in the radial direction is decreased, when the vibration plate is vibrated up and down, the vibration plate is more likely to be brought into a tensioned state. When the vibration plate is brought into a tensioned state, unusual noise may occur. Accordingly, the radius of curvature of the vibration plate should be limited for maintaining the basic sound quality of the speaker unit. Therefore, the method of increasing the curvature of the edge portion cannot allow the lowest resonance frequency to be decreased below a certain level.
In the third method as described above, the increased mass of the vibration system causes a decrease in the sound pressure. Thus, there is a limit to the weight of the mass of the vibration system for maintaining the performance of the speaker unit. Therefore, the method of increasing the mass of the vibration system cannot allow the lowest resonance frequency to be decreased below a certain level.
In the above-described patent literature, the rigidity of the edge portion is suppressed low, which causes a problem that distortion is increased to produce unusual noise similarly to the case in above-described first method. Therefore, in the above-described patent literature, the lowest resonance frequency cannot be decreased below a certain level.